A variation of the collector current (Ic) due to the variation of base-collector voltage and base-emitter voltage are called the Early effects. The Early effects are related to the modulations in the base width of the BJT arising from bias voltages applied to the collector-base junction and base-emitter junction. The direct or forward Early effect corresponds to the base width modulation due to the collector-base voltage variation and the reverse Early effect corresponds to the base width modulation due to the emitter-base voltage variation. The Early effects have particular effects in bandgap circuits which use two or more BJT to generate a voltage output. In such circuits, the impact of the direct and reverse Early voltages contribute to the overall output of the circuit as the output is a combination of the base emitter voltages plus a proportional to absolute voltages (PTAT) based on a base-emitter voltage difference of two bipolar transistors operating at different collector current density. This is more important in silicon based temperature sensors.
There continues to exist a need to compensate for the Early effect.